The Solaras are pretty easy to do. Pry the plastic trim around the radio away from the dash. There are clips that hold it in. One you get one side started it comes out pretty easy, just kind of pops loose. Make sure you take out the ash tray and disconnect the wiring for the clock and cigarette lighter and the hazards lights. Then you can get the trim piece off and out of the way. The radio will be held in with four bolts. Remove those and slide the radio out a little ways, just enough to get the antenna and power wires undone. Take the brackets off the radio, you'll need them to bolt onto the aftermarket HU. Pretty much its plug and play. Get a wiring harness for the Solara and match up the speaker wires from the radio to the new harness and plug it in. It will bypass the stock amplifier and just use the power the new HU has. Its really pretty darn easy to put a new radio in these cars. I would suggest checking fader and balance controls before bolting it back in, to make sure you dont wire something wrong. Bolt the factory brackets onto your radio and install pocket right below the radio. It will just kind of fit into place between the part of the bracket that hangs down below the radio. (You'll need to buy the pocket separately) After you get it bolted back in. Hook your hazard control and all the other wires you disconnected from the trim piece and snap it back in.
Of course its a little detailed than this once you start to do it, but really its very very easy....
Also
www.the12volt.com has couple articles on The Solara and Toyotas in general..